This invention relates generally to a signal transmission apparatus of two-wire type, and more particularly to a signal transmission apparatus of two-wire type suitable for transmitting an output signal of a detector, such as for example a pH meter, a detector of the concentration of dissolved oxygen in a liquid, and the like, for the purpose of remote measurement.
In the case of transmitting an output signal of a detector, such as a pH meter, a detector of the concentration of dissolved oxygen in a liquid and the like, arranged in a place of detection (the transmitting side) to a indicating or recording device arranged in a place of indication or recording (the receiving side) remote from the place of detection it has been a common practice hitherto to use a signal transmission apparatus of four-wire type in which a transmitting circuit arranged in the place of detection and a receiving circuit arranged in the place of indication or recording is connected by four-wire transmission line and two wires of the transmission line are used to supply operating power of the transmitting circuit from the receiving circuit and remaining two wires are used to transmit the output signal of the transmitting circuit to the receiving circuit.
However, the signal transmission apparatus of four-wire type which has been used hitherto has the following detects or disadvantages:
One defect or disadvantage is the fact that it is very difficult to obtain stable operation of the transmitting and receiving circuits, because the input side and the output side of the transmitting circuit are not insulated for DC voltage and it is impossible to ground the transmitting circuit and the receiving circuit independently.
Another defect or advantage is the fact that the construction of a four-line transmission line is unavoidably complicated.